1. Field of the Invention
The present invention relates to a motor in which rotor length in the direction of a rotation axis is shortened, and a light amount adjusting apparatus in which this motor is used as a driving force for a blade member such as a shutter or an f-stop number adjusting mechanism.
2. Related Background Art
In general, operation concerning the exposure of a digital camera is performed as follows.
First of all, a main power source is turned on before taking a picture, a shutter blade is held in an open position when an image pickup element becomes a working condition, and the luminous flux of an objective field reaches the image pickup element. The image pickup element that receives the luminous flux of the objective field performs photoelectric conversion and repeats the accumulation of electric charges, and the emission and transmission of the accumulate charges. Then, an image monitor displays an image of the objective field on the basis of the transmitted signal. When a release button is pushed, an f-stop number and exposure time are determined according to an output of the image pickup element at that time, and the aperture blade is driven according to the determined f-stop number. Next, the image pickup element discharging the accumulated charges is made to start the accumulation of electric charges, and at the same time, an exposure time controlling circuit is started while making an accumulation start signal a trigger signal. When predetermined exposure time elapses, the shutter blade is moved to and held in a closed position for the luminous flux of the objective field to reach the image pickup element. Thereafter, the accumulated electric charges are transferred, and image information is recorded in a recording medium by an image recording apparatus. The reason why exposure to the image pickup element is prevented during transferring the electric charges is to prevent the electric charges from changing during transferring the electric charges by extra light.
A small motor is used as a drive unit that operates these shutter blade and aperture blade. There is a brushless type motor as a suitable form for a small motor, and further, there is a stepper motor, shown in FIG. 9, as the brushless type motor.
This is constituted by coaxially winding a coil 105 around a bobbin 101, and the bobbin 101 is axially held and fixed with two stator yokes 106. In addition, stator teeth 106a and 106b are staggered in an inner circumferential face of the bobbin 101 in the stator yoke 106. In a housing 103, a stator 102 is constituted by fixing the stator yoke 106 that is integrated with the stator teeth 106a or 106b. In one of two housings 103, a flange 115 and a bearing 108 are fixed, and in another housing 103, another bearing 108 is fixed. A rotor 109 consists of rotor shaft 110 and a rotor magnet 111 fixed to this, and there is a gap between the rotor magnet 111 and stator yoke 106a of the stator 102. Then, the rotor shaft 110 is supported rotatably by two bearings 108.
However, since such a type of motor that is shown in FIG. 9 is provided while centering on an output shaft, the motor does not have hollow structure. Therefore, when this motor is used as a driving force that drives an aperture blade, a shutter, a lens, or the like, this motor is arranged so that this motor may become parallel to an optical axis outside the lens in a body tube of a camera. Hence, the radius of the body tube becomes a value obtained by adding the diameter of the motor to the radius of the lens and the radius of an aperture opening.
Therefore, this applicant proposed a motor that was lessened in the radial direction of the rotor in U.S. Pat. No. 5,831,356. Since this motor was miniaturized in the radial direction of the rotor, it was possible to make the diameter of the body tube small enough even if the motor was arranged in the outer circumferential face of the lens.
On the other hand, another motor is proposed, the motor that has a hollow shape and is constituted so that the luminous flux of an objective field can pass the hollow portion. Since the luminous flux of the objective field passes a core of the motor, this is more suitable for the miniaturization in the radial of the body tube than the structure that the motor is arranged in parallel with the lens. Moreover, if motor length in the direction of a rotation axis of the rotor is shortened, it is possible to constitute the lens body tube very compactly.
In addition, motors with short total length in the rotation axis of the rotor are proposed in Japanese Patent Application Laid-Open No. 7-213041, Japanese Patent Application Laid-Open No. 2000-50601, and the like. Though the motors described in the above-mentioned two gazettes are not hollow, it will be possible to modify them into hollow motors by providing each ring rotor, making this rotor an output member, and providing a hole in a central portion of its housing.
The simplified structure of the motors described in the above-mentioned gazettes is shown in FIGS. 10 and 11. A motor having the simplified structure is constituted by a plurality of coils 301, 302, and 303, and a disc-like magnet 304, a coil has a thin coin shape as shown in figure, and a pivot of the coil is arranged in parallel to a magnet shaft. The disc magnet is magnetized in the axial direction of the disk, and is arranged so that a magnetized side of the magnet may face the coil. The magnet 304 is rotated by energizing a plurality of coils 301 to 303 sequentially and generating magnetic flux. Since the coils 301 to 303 and magnet 304 are formed thinly together, it is possible to lessen the size in the direction of the rotation axis of the motor.
Nevertheless, in the motors proposed in Japanese Patent Application Laid-Open No. 7-213041, Japanese Patent Application Laid-Open No. 2000-50601, and the like, as shown in FIG. 11, the magnetic flux generated in each coil did not necessarily effectively go to the magnet 304, but some magnetic flux went in the direction out of the magnet. Therefore, torque was not so large for the size of the entire motor.
In addition, since the coils 301 to 303, and the magnet 304 are abreast arranged in the direction of the rotation axis of the motor, the length of the motor in the axial direction becomes the length obtained by adding the height of the coil to the height of the magnet at lowest. When this motor was used as an adjustment mechanism of the shutter blade or the aperture blade, the total length of the apparatus did not become so short, and hence, this motor was not so suitable for the structure that an image-taking lens is arranged near the aperture blade or the shutter blade. Therefore, in order to provide a motor where the total length in the direction of the rotation axis is further shortened and torque is improved, it is preferable to further improve the motor.